1. Field of the Invention
The present invention relates to a process for producing oil fractions by liquefying coals, particularly non-caking coals such as brown coal and sub-bituminous coal. More particularly, it relates to an improved process for producing oil fractions from such coals by a two-step hydrogenation method.
In general, when coal is heated together with a hydrocarbon solvent under hydrogen pressure, condensed polynuclear aromatic compounds which comprise the coal undergo hydrogenation, which results in the solid coal becoming soluble in a solvent. Then, after the removal of non-dissolved coal which remains in a solvent-insoluble form because it does not undergo hydrogenation and the ash content of the coal by a separation operation such as filtration, if the solvent is distilled off, a heavy liquefied product (hereinafter referred to simply as "CLB" i.e. Coal Liquid Bottom) can be obtained. From CLB, it is possible to produce oil fractions such as naphtha or gasoline by subjecting CLB to a further treatment such as hydrogenation. Various methods are being studied to accomplish this objective. The coal reserves in the world are estimated to be about 10.sup.13 tons, and about 25% of these reserves are said to be low quality coals of low coalification rank such as brown coal which has a low utilization value. In the recent past it has become extremely important to be able to modify or convert such brown coal or sub-bituminous coal efficiently into oil fractions which have a wide range of applications, not to mention the problem of petroleum shortage.
Research for the production of oil fractions from coals was extensively conducted in Germany prior to the second World War, then declined as the petroleum era started, and recently regained its importance. Various studies are being made in the United States, West Germany and various other countries in the world, and various techniques for the liquefaction of coals have been proposed. The techniques so far studied may be generally classified into single step processes and two step processes. The single step processes include SRC II process of Gulf Company, U.S.A., EDS process of Exxon, H-Coal process of Hydrocarbon Research Institute, and New IG process of Rule Coley Co., West Germany. In these single step processes, hydrogenation is conducted in a single step in the presence or absence of a fluidized catalyst at a high temperature and under a high pressure. However, they have various drawbacks such as that the selectivity of conversion to the desired oil fractions is rather low and the apparatus which are used are expensive since the reaction is conducted under severe conditions at a high temperature in a high pressure atmosphere.
On the other hand, International Coal Refining Co. and Lummus Co., in U.S.A., have proposed their own two step processes for liquefaction, and Chevron Co. has proposed a three step process for liquefaction, and these processes are being developed. In these processes, the first hydrogenation is conducted in a relatively short period of time under relatively high temperature and high pressure conditions in the presence or absence of a fluidized catalyst until the coal is liquefied or until a solvent refined coal is obtained, and then after separating the ash content, the catalysts and the solvent from the hydrogenation reaction product, the second hydrogenation reaction is conducted in the presence of a catalyst.
As compared with the single step process, these multi-step processes have advantages such as that the reaction conditions are mild, at least part of the ash content, heavy metals and the like is removed, whereby the effective life of the catalysts for the second hydrogenation can be prolonged, and the selectivity for the desired product can be improved. However, in the conventional two step processes for liquefaction, the yield of the oil fraction from the coal has not yet reached a satisfactory level. If an attempt is made to increase the yield, the reaction conditions which are required to do this are severe particularly in the second hydrogenation reaction, which leads to deactivation of the secondary hydrogenation catalysts, as well as an increase of the costs.
The present inventors have conducted extensive research to overcome these difficulties which are inherent in the conventional processes and to provide a process for liquefying coal, which is economically feasible. As a result, it has been found that the preasphaltene components are the major factor for the deactivation of the catalyst in the second hydrogenation step, and that it is possible to moderate not only the second hydrogenation reaction conditions, but also the first hydrogenation reaction conditions and maximize the overall yield of the oil fraction from the coal. The results of the invention are achieved by controlling the amounts of the preasphaltene components and by controlling the conversion of the heavy oil components in the deashed liquefied oil which is supplied to the second hydrogenation reaction so that a substantial amount of heavy oil components is present in the secondary hydrogenation reaction product. A further aspect of the invention is the recycling of the heavy oil components to the first hydrogenation reaction zone. Thus, it has been found possible to realize an industrially advantageous process for converting coal to the oil fraction. The present invention is based on these discoveries.
Namely, it is an object of the present invention to provide a method of converting coal to an oil fraction, which is industrially extremely valuable, by substantially improving the yield of the oil fraction from the coal. Such an object can readily be attained by a process for converting coal to an oil fraction, which comprises subjecting coal to a first hydrogenation reaction, deashing the reaction product of the first hydrogenation reaction and subjecting the deashed liquefied oil to a second hydrogenation reaction. In the process coal, a solvent and hydrogenated heavy oil components are supplied to the first hydrogenation reactor; at least a part of the oil fraction from the first hydrogenation reaction product is obtained; a substantial amount of preasphaltene components is removed simultaneously with or independently of the deashing operation from the first hydrogenation reaction product, and the deashed liquefied oil containing heavy oil components and not greater than 20% by weight of preasphaltene components thereby obtained, is supplied to the second hydrogenation reactor; an oil fraction and heavy oil components are separated from the second hydrogenation reaction product, and the heavy oil components are recycled to the first hydrogenation reaction in an amount of at least 20% by weight relative to the heavy oil components in said deashed liquefied oil.